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First published online June 25, 2007
doi: 10.1242/10.1242/dev.02864
1 División de Genética, Universidad Miguel Hernández,
Campus de San Juan, Ctra. de Valencia s/n, 03550-San Juan de Alicante,
Spain.
2 Instituto de Biología Molecular y Celular de Plantas (CSIC-UPV), Avda.
de los Naranjos s/n, 46022-Valencia, Spain.
Author for correspondence (e-mail:
laborda{at}umh.es)
Accepted 25 April 2007
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SUMMARY |
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 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key words: Arabidopsis, Fruit development, Pattern formation, ASYMMETRIC LEAVES 1, BREVIPEDICELLUS (KNAT1), Class I KNOX genes
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INTRODUCTION |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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; Bowman et al.,
1999; Dinneny and Yanofsky,
2005; Balanzá et al.,
2006). The ovary is composed of two valves connected on both sides
by the replum (Sessions and Zambryski,
1995). After pollination, Arabidopsis develops a silique,
the characteristic dehiscent fruit shared by all Brassicaceae
(Ferrándiz et al.,
1999). The valve margin, a thin region of small cells located
between replum and valve tissues, plays a crucial role in dehiscence
(Ferrándiz, 2002).
The MADS-box gene FRUITFULL (FUL) represses the
expression in valves of genes involved in valve margin development
(Ferrándiz et al.,
2000a; Liljegren et al.,
2004), the MADS-box genes SHATTERPROOF (SHP1 and
SHP2) (Liljegren et al.,
2000), and their downstream genes ALCATRAZ (ALC)
and INDEHISCENT (IND), both of which code for basic
helix-loop-helix (bHLH) domain proteins
(Rajani and Sundaresan, 2001;
Liljegren et al., 2004). In
this way, FUL prevents valves from adopting a valve margin fate
(Ferrándiz et al.,
2000a; Liljegren et al.,
2004). The FUL and SHP genes are induced by the
cooperating activities of FILAMENTOUS FLOWER (FIL)
(Chen et al., 1999;
Sawa et al., 1999a;
Sawa et al., 1999b) and
YABBY3 (YAB3) (Siegfried
et al., 1999), two genes belonging to the YABBY family involved in
abaxial tissue specification, and JAGGED (JAG), a gene that
encodes a putative transcription factor with a single C2H2 zinc-finger domain
and promotes growth in lateral organs
(Dinneny et al., 2004;
Ohno et al., 2004). These
genes probably act in a concentration-dependent manner, in such a way that
activation of FUL would require high levels of their products, while
SHP expression would be induced by lower levels
(Dinneny et al., 2005). The
homeobox gene REPLUMLESS (RPL) downregulates valve margin
genes in the replum (Roeder et al.,
2003; Liljegren et al.,
2004) by repressing the expression of FIL, YAB3 and
JAG (Dinneny et al.,
2005). This gene, also designated BELLRINGER
(BLR), PENNYWISE (PNY) and VAAMANA
(VAN), interacts with the class I KNOTTED1-like homeobox
(KNOX) genes SHOOTMERISTEMLESS (STM),
BREVIPEDICELLUS (BP, also known as KNAT1) and
KNAT6 to regulate meristem function
(Byrne et al., 2003;
Smith and Hake, 2003;
Bhatt et al., 2004).
The mechanism by which leaf founder cells are distinguished from stem cells
of the meristem involves downregulation at positions of leaf initiation of
class I KNOX genes (Lincoln et al.,
1994; Dockx et al.,
1995; Long et al.,
1996; Semiarti et al.,
2001), and the subsequent expression of the ASYMMETRIC
LEAVES genes (AS1 and AS2)
(Byrne et al., 2000;
Byrne et al., 2002).
AS1 codes for a myb transcription factor
(Byrne et al., 2000;
Sun et al., 2002), and
AS2 encodes a protein containing the LATERAL ORGAN BOUNDARIES domain
(Iwakawa et al., 2002;
Shuai et al., 2002). Both
AS genes interact in the same pathway to promote the differentiation
of leaf cells by maintaining the repression of BP, KNAT2 and
KNAT6. Thus, in the absence of any of the AS products, these three
KNOX genes are misexpressed in leaves
(Byrne et al., 2000;
Ori et al., 2000;
Semiarti et al., 2001;
Xu et al., 2003).
As early as 1790, Goethe advanced the hypothesis that floral organs are
modified vegetative leaves (Coen,
2001). This hypothesis has found strong support in genetic and
molecular research carried out during the last 15 years, such as the
transformation of floral organs into carpelloid leaf-like organs in a triple
mutant lacking the ABC homeotic functions
(Bowman et al., 1991), and the
transformation of vegetative leaves into floral organs by the ectopic
expression of SEPALLATA and floral homeotic genes
(Honma and Goto, 2001;
Pelaz et al., 2001).
Interestingly, AS genes are also expressed in carpels
(Byrne et al., 2000;
Sun et al., 2002). However,
despite the probable foliar evolutionary origin of this organ
(Friedman et al., 2004), the
role of AS1 and AS2 in the downregulation of class I KNOX
genes in carpels remains unclear. In this report, we show that AS1
also negatively regulates BP in ovary tissues. Thus, in as1
mutants, BP is misexpressed in the ovary, causing an increase in
replum size and a slight reduction in valve width. We also show a strong
interaction between loss-of-function alleles in AS1 and FUL,
so that double mutants exhibit very large repla and a small valve region.
These phenotypes can be interpreted as a shift of valve margins to more
lateral positions in the ovary. Our results, showing that BP is
expressed in the replum but not in valves and that this gene positively
regulates RPL expression, together with the phenotype caused by
BP misexpression, strongly suggest that class I KNOX genes play a
crucial role in replum development. A model is presented that accounts for the
function of these and other genes in patterning the ovary.
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MATERIALS AND METHODS |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) and
rpl-2 (pny-40126)
(Roeder et al., 2003;
Smith and Hake, 2003), both in
Col background; as2-1 (Redei,
1965), in An background; bp-1
(Koornneef et al., 1983;
Venglat et al., 2002), in
Ler background; and transgenic lines 35S::BP
(Chuck et al., 1996) in No-0
background, and KNAT1::GUS-1 (Ori
et al., 2000) in Col background. The KNAT1::GUS-18 and
as1-1 KNAT1::GUS-18 lines (Ori et
al., 2000), both in Col background, were provided by Sarah Hake
(Plant Gene Expression Center, Albany, CA) and Naomi Ori (The Hebrew
University of Jerusalem, Israel) and the bp-9 rpl-2 (bp-9
pny-40126) double mutant (Smith and
Hake, 2003), in Col background, was provided by Sarah Hake. The
knat2 gene trap line GT7953 (Byrne
at al., 2002) and the BLR::GUS line
(Byrne et al., 2003), both in
Ler background, were provided by Robert Martienssen (Cold Spring
Harbor Laboratory, NY). The rpl-1 mutant
(Roeder et al., 2003), in
Ler background, and the transgenic line SHP2::GUS
(Savidge et al., 1995;
Roeder et al., 2003), in No-0
background, were provided by Martin Yanofsky (University of California at San
Diego, La Jolla, CA). The ful-1 mutation is in Ler
background (Gu et al., 1998).
The as1-104 allele has been isolated during a genetic screen of ethyl
methanesulphonate (EMS) mutagenized ful-1 plants
(Roeder et al., 2003). Plants
were grown at 20-22°C with continuous cool-white fluorescent light as
previously described (Ripoll et al.,
2006).
Plant genetics
Multiple mutants were identified among the F2 from the
characteristic mutant phenotype caused by individual mutations: leaf phenotype
for as alleles, inflorescence phenotype for rpl alleles,
fruit phenotype for ful-1, and the downward-pointing fruit phenotype
for bp-1. The wild-type KNAT2 allele was genotyped using two
primers, KNAT2-1F (GACGTTTCAGTGTCGTACTGG) and KNAT2-1R
(CAAGCCTCTTGGCCATCAAGC), flanking the Ds transposon. The
knat2 homozygous plants did not yield any PCR product, and all their
offspring showed resistance to kanamycin. Partial introgression in Col
background of the 35S::BP construct and the as1-1 bp-1
genotype was achieved by crossing twice to Col and as1-1,
respectively, to obtain 35S::BP (2xCol) and as1-1 bp-1
(2xCol) plants.
Student t-tests were performed on the data set of Fig. 2. In every case, the null hypothesis (Ho) to be tested was that the lines being compared showed the same phenotype. Tests of statistical significance are included in the supplementary material (see Table S1 in the supplementary material).
Microscopy
Light microscopy and scanning electron microscopy (SEM) were performed as
previously described (Ripoll et al.,
2006). For GUS staining, samples were treated for 15 minutes in
90% ice-cold acetone, and then washed for 5 minutes with washing buffer (25 mM
sodium phosphate; 5 mM potassium ferrocyanide; 5 mM potassium ferricyanide; 1%
Triton X-100), vacuum infiltrated for 10 minutes in staining buffer (washing
buffer with 2 mM X-Gluc) and incubated overnight at 37°C. Phloroglucinol
staining was done as previously described
(Liljegren et al., 2000).
In situ hybridization was carried out as described by Ferrándiz and
co-workers (Ferrándiz et al.,
2000b). A 369 bp fragment from AS1 was amplified by PCR
with the primers AS1-7 (GTAGCGAGAGTGTGTTCTTGTC) and AS1-8
(CAGGGGCGGTCTAATCTGC) and cloned into the pGEM-T vector (Promega). Two
micrograms of NcoI-linearized plasmid were used to generate a
DIG-labeled antisense riboprobe. A sense DIG-labeled riboprobe was generated
after digestion with SpeI.
Quantitative real-time PCR
RNA was extracted using the Qiagen RNeasy Plant Minikit, and DNA
contamination was removed using the Qiagen RNase-free DNase Set.
Reverse-transcription was performed from 1 µg RNA using the SuperScript
First Strand kit (Invitrogen). Real-time PCR was performed using the Sybr
Green PCR Master Mix (Applied Bioscience) in a volume of 20 µl on an ABI
Prism 7000 System (Applied Bioscience). ELONGATION FACTOR 1- 
(EF1-, AT5G60390) was used as an internal control to normalize
for variation in the amount of cDNA template
(Frigerio et al., 2006).
Primers RPL-F (AAGGGCTTGGCTCTTCGATC) and RPL-R (TCTGTATCTGTTGGATAAGGATGCA)
were used to amplify a 51 bp fragment from RPL cDNA. The reported
values are averages of two biological replicates, each one composed of three
technical replicates. To calculate relative expression levels, RPL
transcript levels were normalized relative to the standard EF1-
using the equation 
CT=CT(RPL) -
CT(EF1-). Relative expression levels were
calculated applying the formula 2-[CT
(35S::BP) -CT (No-0)].
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RESULTS |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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; Dinneny et al.,
2005; Balanzá et al.,
2006). In an attempt to identify new functions involved in fruit
patterning, we studied the possible role in this process of genes that
determine important functions in leaf development. Two such candidates are
AS1 and AS2, the products of which physically interact to
prevent the expression in leaves of the class I KNOX family genes BP,
KNAT2 and KNAT6 (Byrne et
al., 2000; Ori et al.,
2000; Semiarti et al.,
2001; Byrne et al.,
2002; Xu et al.,
2003). We compared fruits of as1-1 and Col, and found that the mutant displayed slightly altered siliques with a moderate bumpy appearance (Fig. 1A,B). A closer inspection of these fruits showed that they had developed enlarged repla (Fig. 1C,D). Cross sections revealed that mutant repla contained more cells than those of the wild type, as seen in the outer layer of the replum (Fig. 1E,F; Fig. 2; see Table S1 in the supplementary material). Unlike the case in the replum, the number of outer epidermal cells in mutant valves was smaller (Fig. 2; see Table S1 in the supplementary material), causing a reduction in valve width, which was the most likely reason for the bumpy aspect of the fruits. This phenotype was observed irrespective of the genetic background, as the as1-104 mutant, homozygous for a null allele of AS1 in Ler background (see Fig. S1 in the supplementary material), also showed large repla and a reduction in valve width (Fig. 1G-L; Fig. 2; see Table S1 in the supplementary material). In addition, fruits of as2-1 displayed the same phenotype as seen in those of as1-1 (Fig. 1M), suggesting that both genes collaborate in the same pathway of fruit patterning.
|
). However, we observed no defects in the dehiscence
of as1-1 siliques, and phloroglucinol staining of cross sections of
these fruits showed normal valve margins and unaffected lignification patterns
(Fig. 1Q,R). Consistent with
these phenotypes, GUS staining from a SHP2::GUS reporter in the
as1-1 background exhibited a pattern similar to that seen in the wild
type, although the increased size of the replum was clearly highlighted
(Fig. 1S-U).
BP is involved in the fruit phenotype of as1 mutants
Misexpression of class I KNOX genes appears as a possible cause for the
fruit phenotypes observed in as1-1 and as2-1 mutants. Thus,
we examined the effect on the fruit of the 35S::BP construct, in the
original No-0 background and after its partial introgression in Col
[35S::BP (2xCol) plants]. These plants displayed a phenotype similar
to that seen in as1-1 fruits (Fig.
1N; see Table S1 in the supplementary material). Repla were wider
than those of wild-type plants and showed an increased cell number, while the
valves exhibited a small reduction in size because of the lower number of
cells (Fig. 2; see Table S1 in
the supplementary material). Numbers of cells in valves and repla of wild-type
segregants of the introgression in Col of the 35S::BP construct
(valve=63±3.9, n=20; replum=8±0.9, n=20) were
also clearly different from those seen in 35S::BP and
35S::BP (2xCol) plants (see Table S1 in the supplementary
material).
To further investigate the role of BP in the mutant phenotype, we
obtained double mutants carrying as1-1 and the null allele
bp-1, which were inspected after two backcrosses with Col [as1-1
bp-1 (2xCol) plants], as well as as1-104 bp-1 plants in
Ler background. The alteration produced in fruits by both
as1 mutations was partially alleviated in the double mutants. Repla
were narrow, practically reverting to the appearance of the wild type
(Fig. 1O,P), and numbers of
cells in both repla and valves were different from those of the as1
mutants (Fig. 2; see Table S1
in the supplementary material). This partial rescue suggests that factors
redundant with BP are also involved in the fruit mutant phenotype of
as1-1, and the other class I KNOX genes can be considered good
candidates for such a redundant activity. A possible candidate is the
KNAT2 gene, which is expressed in the wild-type replum
(Ori et al., 2000;
Pautot et al., 2001). However,
the knat2 allele did not modify the phenotypes of as1-1 and
as1-1 bp-1 fruits (see Fig. S4E-H in the supplementary material).
This suggests that either KNAT2 does not participate in the fruit
mutant phenotype conferred by the as1-1 allele or that KNAT2
is completely redundant with another class I KNOX family gene.
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; Sun et al.,
2002). Therefore, we first studied the expression of AS1
in wild-type fruits by in situ hybridization. High levels of AS1
transcripts were detected in valves, and low levels in the replum
(Fig. 3A,B; see Fig. S2A-D in
the supplementary material). This result is consistent with the phenotype of
as1-1 fruits, where both replum and valve tissues were altered.
|
). During
wild-type gynoecium development, the KNAT1::GUS-18 reporter was
expressed in a stripe of cells that would develop into the replum and valve
margin, with the highest levels detected at stage 12
(Fig. 3C,E). This expression
was conserved during later stages as fruits developed
(Fig. 3G). Beginning at stage
12, strong expression was also detected in the style
(Fig. 3C; see Fig. S2E in the
supplementary material). Examination of KNAT1::GUS-18 expression in
as1-1 showed that the reporter activity occurred in a broader domain
in the presumptive replum (Fig.
3D) and was ectopically observed, being detected in all tissues of
as1-1 ovaries, including the valves
(Fig. 3D,F). This result
accounts for the participation of BP in the mutant phenotype of
as1-1 fruits, and the similarity of the fruit phenotype observed
between as1-1 and 35S::BP plants. In addition, the use of
the KNAT1::GUS-1 transgene revealed a conspicuous increase in GUS
staining intensity in the replum of as1-1 fruits
(Fig. 3H,I). This finding
strongly suggests that BP expression in the replum is higher in the
as1-1 mutant than in the wild type.
|
), binds the class I KNOX transcription factors BP, STM and
KNAT6 to form heterodimers that regulate meristem function
(Byrne et al., 2003;
Smith and Hake, 2003;
Bhatt et al., 2004). Therefore,
a reasonable hypothesis is that BP and RPL also interact in
the replum.
We used a reporter line for RPL, the BLR::GUS line
(Byrne at al., 2003), to
examine the expression of the gene in wild-type and 35S::BP plants.
In order to compare both expressions in homogeneous genetic backgrounds, we
studied GUS staining in two kinds of F1 individuals carrying the
BLR::GUS construct in heterozygosis, those resulting from a cross
between BLR::GUS and 35S::BP plants
(35S::BP/+;BLR::GUS/+ plants) and those
resulting from a cross between BLR::GUS and No-0 plants
(+/+;BLR::GUS/+ plants). RPL
expression in the wild type was confined to the replum, with the strongest
signal at stage 12 (Fig. 4A),
as previously reported (Roeder et al.,
2003). Plants containing the 35S::BP construct exhibited
ectopic expression of BLR::GUS in cotyledons, leaves, valves and
valve margins (Fig. 4A-D),
which indicates that BP positively regulates the expression of the
RPL promoter. This activation was confirmed by quantitative real-time
PCR (qRT-PCR), which showed an increase of RPL transcripts in
35S::BP plants compared with the No-0 accession (see Fig. S3 in the
supplementary material).
|
). In
accordance with the strong mutant phenotype for meristem function previously
reported in bp-9 rpl-2 plants
(Smith and Hake, 2003), fruits
of this double mutant exhibited a more severe replumless phenotype than
rpl-2 (Fig. 4G),
similar to that shown by the strong rpl-3 mutant
(Roeder et al., 2003),
suggesting that BP and RPL also interact in the replum.
However, fruits produced by bp-1, knat2 and bp-1 knat2
plants showed a wild-type aspect, both in replum and valves (see Fig. S4A-D in
the supplementary material), which indicates that the activity of these genes
is not indispensable for RPL function and replum development,
probably due to the redundant activities of other class I KNOX genes
(Byrne at al., 2002). We then obtained as1 rpl double mutants to investigate whether the overexpression of class I KNOX genes affects the replumless phenotype caused by rpl alleles. Thirteen out of 24 repla of the as1-1 rpl-2 double mutant exhibited a wild-type phenotype (Fig. 4H,L), whereas the remaining 11 showed a moderate mutant phenotype (not shown), indicating a partial rescue of rpl-2 repla by as1-1. The as1-1 allele also rescued replum development when combined with rpl-1. Thus, outer repla of the as1-1 rpl-1 double mutant, both in ER and er backgrounds, displayed either a wild-type (Fig. 4I) or a moderate mutant (Fig. 4J) phenotype. This suggests that, in the absence of RPL, an excess of class I KNOX products may prevent, either directly or indirectly, the expression of valve margin identity genes in the replum. In addition, the number of outer epidermal cells in valves of as1-1 rpl-2 fruits (48.3±5.2; n=18) was similar to those of as1-1 and 35S::BP plants (Fig. 2), indicating that RPL plays no role in the reduced valve width of these plants.
Synergistic interaction between as1 and ful mutant alleles
BP is expressed in the presumptive replum and valve margin, and
might have a role in controlling pattern formation in these tissues. In this
sense, the fruit phenotype caused by the as1-1 mutation and the
resulting overexpression of BP could be interpreted as a lateral
shift of the borders between the territories of the replum and the valves in
the ovary, which would result in replum expansion, a consequent change in the
positions of the valve margins, and a modest reduction in valve size.
According to this hypothesis, eliminating FUL, a gene important for
valve development, in a background that overexpresses BP should
result in a synergistic interaction, severely affecting both replum and
valves, owing to a greater shift of the borders.
After pollination, ful-1 fails to appropriately differentiate and
elongate its valve cells, because of the ectopic expression of valve margin
identity genes (Ferrándiz et al.,
2000a; Liljegren et al.,
2004). Consequently, mutant siliques are small in size and show
compressed and creased repla (Fig.
5A,C), a phenotype that can also be interpreted in terms of a
shift in the boundaries between valves and replum, giving rise to the small
valves and large repla of ful-1. As predicted, the as1-104
ful-1 double mutant displayed extremely small valves, and very large,
rough and distorted repla, indicating a strong enhancement of the phenotypes
of the two single mutants both in valves and replum
(Fig. 5B,D), and favoring the
hypothesis that BP overexpression affects the positioning of the
borders between valves and replum. The same phenotype was also observed in
as1-1 ful-1, 35S::BP ful-1 and as2-1 ful-1 fruits (see Fig.
S5 in the supplementary material).
As the ful-1 mutation is caused by a Ds transposon
carrying a GUS enhancer trap element that has a transcription pattern that
mimics the expression domain of the FUL gene, we studied the GUS
expression pattern driven by the FUL promoter in ful-1,
as1-104 and as1-104 ful-1 fruits. Expression of the FUL
enhancer trap in the ful-1 single mutant was restricted to the valve
region (Fig. 5F), as previously
reported (Gu et al., 1998),
and the same expression pattern was detected in as1-104 plants that
carried the ful-1 allele in heterozygosis
(Fig. 5E). This expression
remained unchanged in double mutant siliques in which GUS staining was also
detected in the aberrant valves (Fig.
5G). Interestingly, the comparison of GUS staining in the single
and double mutants clearly showed the different sizes of valves and repla. The
valve region was much more reduced in as1-104 ful-1 than in the
single mutants, while the opposite occurred with the replum, which was much
larger in the double mutant (Fig.
5E-G).
As shown above, RPL does not contribute to the reduction in cell numbers in the valves of as1-1 and 35S::BP siliques, as the number of outer epidermal cells in valves of these fruits is similar to those of as1-1 rpl-2 plants. However, this observation does not exclude the possibility that RPL might participate in the fruit phenotype caused by as1 null alleles in the absence of FUL function. To examine this, we crossed the as1-1 rpl-1 double mutant, in ER background, with ful-1 to obtain the as1-1 ful-1 rpl-1 triple mutant in both ER and er backgrounds. Siliques from these plants exhibited a more moderate mutant phenotype, both in valves and replum, than those of the as1-104 ful-1 double mutant (Fig. 5H,I). This result indicates that RPL participates in the strong phenotype of as1 ful-1 and 35S::BP ful-1 siliques, along with BP and other class I KNOX genes.
|
DISCUSSION |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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AS function represses BP in the gynoecium
The mechanism involved in patterning the ovary shows interesting
similarities to events that occur at the shoot apex to pattern the apical
meristem and lateral organs. In the gynoecium, the activities of FIL,
YAB3 and JAG promote valve and valve margin development, while
RPL represses the expression of these genes in the replum, ensuring
the formation of this tissue (Dinneny et
al., 2005). In the shoot apex, the antagonistic activities of
meristematic genes and lateral organ-expressed genes allow meristem
maintenance, restricting organogenesis to the organ primordium. Thus,
RPL is expressed in the meristem, where its product binds most class
I KNOX proteins (STM, BP and KNAT6) to regulate developmental processes
(Byrne et al., 2003;
Smith and Hake, 2003;
Bhatt et al., 2004), whereas
FIL, YAB3 and JAG are exclusively transcribed in lateral
organs. Interestingly, several class I KNOX genes are expressed in the replum
(this work) (Long et al.,
1996; Pautot et al.,
2001), a tissue that seems to have meristematic properties,
because it gives rise to the placenta, where ovules are produced. All this
suggests that the replum displays some kind of meristematic attributes
(Roeder and Yanofsky, 2006),
while the valves are more related to leaf blades.
The AS genes are expressed in leaf primordia, where they repress
BP, KNAT2 and KNAT6
(Byrne et al., 2000;
Ori et al., 2000;
Semiarti et al., 2001), but
not the related STM gene, which, in turn, negatively regulates
AS1 and AS2, so that neither of these genes is transcribed
in the meristem (Byrne et al.,
2002). Similarly, in the gynoecium, AS1 is expressed in
valves, where it also represses BP, which is transcribed only in the
replum and valve margin. Thus, as1 alleles cause ectopic expression
of BP in valves, giving rise to an abnormal fruit phenotype.
Accordingly, plants carrying the 35S::BP transgene display this same
phenotype, in such a way that overexpression of the BP gene alone
could account for the As1- fruit phenotype. Nevertheless, removal
of BP function in the as1 background does not completely
rescue the mutant phenotype, suggesting that other class I KNOX genes may also
be misexpressed in as1 pistils. Mutations in AS2 produce the
same fruit phenotype as as1 alleles, suggesting that this gene
interacts with AS1 in the pistil to repress class I KNOX genes, as it
does in leaves (Byrne at al.,
2002; Xu et al.,
2003). Moreover, AS1 is also expressed in the replum,
although at lower levels, and this seems to be necessary to restrain
BP transcripts below certain levels, as the intensity of GUS staining
in the replum of plants carrying KNAT1::GUS-1 clearly increases in an
as1 background. Although the overlapping expression of AS1
and BP in the replum may appear contradictory with previous studies
carried out in leaves (Byrne at al.,
2000; Ori et al.,
2000), the activities of these two genes are not necessarily
exclusive, as both are expressed in the leaves of several mutants
(Kumaran et al., 2002;
Hay et al., 2006).
Do class I KNOX genes confer replum identity?
Along the mediolateral axis, RPL is the only gene that has so far
been shown to play a role in replum differentiation. However, several multiple
mutant backgrounds lacking RPL function, such as as1 rpl, shp1
shp2 rpl, jag rpl and fil rpl, develop basically normal repla
(this work) (Roeder et al.,
2003; Dinneny et al.,
2005), indicating that this gene is not indispensable for replum
formation. Therefore, there must be other gene function(s) involved in the
elaboration of the basal pattern for replum identity.
Although there are no conclusive data, several lines of argument support
the idea that class I KNOX genes might play this role. First, these genes are
transcribed in the replum, but not in valves. This is the case for
STM (Long et al.,
1996), KNAT2 (Pautot
et al., 2001) and BP (this work). Second, the
overexpression of BP, both in as1 mutants and
35S::BP plants, increases the size of the replum, whereas the valve
territory appears slightly reduced, suggesting that BP promotes
replum development and has an opposing role in valve formation. Third,
BP activates the expression of RPL, a gene that plays a
crucial role in the replum. This function of BP may be redundantly
carried out by other class I KNOX genes, as the expression of RPL is
not affected in a bp mutant background
(Smith and Hake, 2003). And
fourth, BP interacts with RPL in the replum, as the bp-9
rpl-2 double mutant shows a stronger replumless phenotype than
rpl-2.
Despite this putative function of BP in replum development, no
mutant phenotype in this tissue has been found to be caused by a null
bp allele. A likely reason for this behavior is the known functional
redundancy among class I KNOX genes (Byrne
at al., 2002). Thus, BP function in the replum of
bp mutants could be assumed by STM, as their products share
high homology (Byrne at al.,
2002), or by KNAT6, which acts redundantly with
STM in the shoot apical meristem
(Belles-Boix et al., 2006).
Regrettably, the redundancy among the members of this gene family precludes a
functional analysis with loss-of-function mutations, since this strategy
should require the isolation of multiple mutant lines lacking a shoot apical
meristem.
|
).
According to the model, the cooperating activities of FIL, YAB3 and
JAG (FIL/JAG activity) promote the expression of
FUL and SHP genes in the valve and the presumptive valve
margin, respectively, in such a way that high levels of
FIL/JAG activity in the valve would activate FUL
expression, whereas the transcription of SHP genes would require only
a weak FIL/JAG activity present in the valve margin. The FUL
product, in turn, prevents the expression of SHP genes in valves.
This same function is carried out by RPL in the replum through the
negative regulation of FIL, YAB3 and JAG. Thus, by the
action of FUL and RPL, SHP activity is restricted to the
presumptive valve margin (Dinneny et al.,
2005). Moreover, it has been suggested that an unknown replum
factor is involved in the negative regulation of FUL expression
(Liljegren et al., 2004), and
it has been shown that the ectopic expression of FUL inhibits the
differentiation of the outer replum
(Ferrándiz et al.,
2000a), suggesting that FUL negatively regulates replum
genes and/or their products. These data, together with the downregulation of
class I KNOX genes by FIL and YAB3
(Kumaran et al., 2002),
support the notion that there are antagonistic gene activities in replum and
valves.
We now add AS and class I KNOX genes to the model
(Fig. 6). AS1 is
expressed at high levels in valves and at lower levels in the replum, thus
preventing the expression of class I KNOX genes in valves while maintaining
the products of these genes below certain levels in the replum. This function
(AS function in Fig.
6) would be brought about in collaboration with AS2, as
as2 alleles produce the same fruit phenotype as as1
mutations. We propose that the territories of valve and replum become
established by the opposing activities of valve factors
(FIL/JAG activity) and replum factors (class I KNOX genes),
while the valve margin forms in a narrow stripe in which both valve and replum
factors are expressed. Valve factors should be working through a gradient,
with the strongest activity in the middle of the valve, coinciding with the
lateral plane of the ovary, in strong agreement with the role of the
FIL/JAG activity in inducing, by means of a
concentration-dependent mechanism, the expression of FUL and
SHP genes in adjacent domains, valve and valve margin, respectively
(Dinneny et al., 2005). In
addition, we hypothesize that class I KNOX genes would be expressed at the
highest level in the replum, while low levels of FIL and YAB3 proteins should
exert a partial downregulation on this family of genes in the valve margin,
because this repression is known to occur in leaves
(Kumaran et al., 2002). This
model is a variation of the basic French flag model for pattern formation
(Wolpert, 1969), whereby three
territories would be determined by the contribution of the opposing gradients
of two antagonistic factors.
According to our model, in as1 and 35S::BP fruits, class
I KNOX genes become overexpressed in the replum region and are ectopically
transcribed in valves, where they antagonize the FIL/JAG
activity, resulting in a shift in the position of the valve margin along the
mediolateral axis. Moreover, lack of outer replum in 35S::FUL fruits
(Ferrándiz et al.,
2000a), the synergistic relationship between as1 and
ful alleles (this work), and the reduction of the mutant phenotype in
the triple as1 ful rpl with respect to the as1 ful double
mutant (this work) suggest that FUL has an inhibitory role on
RPL, and perhaps on class I KNOX genes as well. The model also
accounts for previous results. For instance, fil mutants show a large
replum, yet FIL is not expressed in this domain
(Dinneny et al., 2005). A
possible explanation is that a fall in FIL/JAG activity
would produce an expansion of the expression of the counteracting replum
genes, causing a shift in valve margin position. In 35S::FUL fruits,
the ectopic expression of FUL would inhibit replum gene function,
allowing FIL/JAG activity to exert its role throughout the
ovary.
This work provides further information on the connection between leaf and
carpel development, through the establishment of the possible functions of
BP and AS1 in fruit patterning. The pleiotropic behavior of
these two genes is founded in their expression in several organs, the
different morphologies of which could be explained by changes in the
regulation of the genes, by different responses of their target genes and/or
by the participation of other interacting genes. This same argument may be
extended to the different contribution of one gene in two species. A recent
work has demonstrated that a rice ortholog of RPL participates in
seed shattering and that a punctual mutation in its regulatory sequence is
involved in loss of seed shattering and domestication of this cereal
(Konishi et al., 2006). Thus,
although the dehiscence zone in the Arabidopsis fruit and the
abscission layer at the base of the rice grain are structures that do not
share the same botanical origin, both require RPL function for their
formation. Understanding the contribution of specific genes in the formation
of different structures will help to unravel the evolutionary relationships
both between organs and between species.
Supplementary material
Supplementary material for this article is available at
http://dev.biologists.org/cgi/content/full/134/14/2663/DC1
|
ACKNOWLEDGMENTS |
|---|
|
Footnotes |
|---|
Present address: Division of Biological Sciences, University of California
San Diego, La Jolla, CA 92093, USA
Present address: Unidad de Reproducción Asistida, Clínica
Vistahermosa, Avda. de Denia 103, 03015-Alicante, Spain
|
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